1. Technical Field
The present invention relates to a battery monitoring apparatus that includes a monitoring circuit for monitoring individual cell voltages of a plurality of battery cells which are connected in series to form a battery pack.
2. Related Art
A battery monitoring apparatus in related art is proposed that includes a monitoring circuit and a low-pass filter (for example, see JP-A-2012-159406). The monitoring circuit is provided with a plurality of pairs of sensing terminals for a plurality of battery cells. The pairs of sensing terminals are associated with the plurality of battery cells of which positive and negative electrode terminals are connected to each of the sensing terminals in a pair via individual connecting lines. The low-pass filter is disposed on the connecting lines.
Each of a plurality of current paths is formed between each of the battery cells and the monitoring circuit via the corresponding connecting lines. In the case where the number of components of the low-pass filter is different between the current paths, the cutoff frequency of the low-pass filter may be varied.
From this, a circuit configuration of JP-A-2012-159406 is designed such that the low-pass filter is configured by an RC (resistor-capacitor) filter composed of a resistor and a capacitor with the same number of components of the low-pass filter between the current paths.
Specifically, the connecting lines on the battery pack side are provided with a plurality of common lines. Each of the common lines is connected to the negative electrode terminal of a high-potential side battery cell of serially connected adjacent battery cells and to the positive electrode terminal of a low-potential side battery cell of serially connected adjacent battery cells.
The connecting lines on the monitoring circuit side are provided with two branch lines into which each of the common lines is branched. One of the two branch lines is connected to a sensing terminal that corresponds to the negative electrode terminal of the high-potential side battery cell. The other of the two branch lines is connected to a sensing terminal that corresponds to the positive electrode terminal of the low-potential side battery cell.
The low-pass filter includes resistors and capacitors. The resistors are located on a pair of branch lines that correspond to a pair of terminals (positive and negative electrode terminals) of the monitoring circuit. The capacitors are located between the pair of branch lines.
The filter configuration disclosed in JP-A-2012-159406 has a feature of sufficiently attenuating differential noise (in-phase fluctuation) generated between the individual connecting lines that connect the battery cells to the monitoring circuit. However, this filter configuration is not able to sufficiently attenuate common mode noise (in-phase fluctuation) generated between the individual connecting lines and a ground line.
This will be specifically described referring to FIGS. 5 to 8. FIGS. 5 and 7 show an example in which a filter circuit 300 in related art is disposed between a battery pack 100 and a monitoring circuit 200. FIGS. 6 and 8 show Bode diagrams for explaining filter characteristics.
The filter circuit 300 in FIG. 5 has filter characteristics of which a transfer function G(s) (=Vo1/Vi1) of an RC filter for a single battery cell 100a is expressed by the following formula F1 and, as shown in FIG. 6, sufficient attenuation is achieved in a high-frequency range in which the frequency f becomes higher than the cutoff frequency fc.G(s)=(Vo1/Vi1)=1/(1+2RCs)  (F1)
The filter circuit 300 in FIG. 7 has filter characteristics of which a transfer function G(s) (Vo2/Vi2) of an RC filter for n battery cells 100a is expressed by the following formula F2 and, as shown in FIG. 8, sufficient attenuation is not achieved in the high-frequency range in which the frequency f becomes higher than the cutoff frequency fc, because resistance components are applied to the output Vo2.(Vo2/Vi2)=[2{(n−1)/n}RCs+1]/(1+2nRCs)  (F2)
In this way, common mode noise is not sufficiently attenuated in the filter configuration disclosed in JP-A-2012-159406, and there is a probability that the common mode noise would enter the monitoring circuit, which leads to the probability of causing malfunction in the monitoring circuit.
In order to cope with common mode noise, a ground-type filter configuration, in which the capacitors are directly grounded for each of the connecting lines, may be used.
In this case, each capacitor for the high-potential side battery cell of the battery cells having a high voltage to ground is required to be configured by a high-voltage capacitor. This raises a problem of drastically increasing the cost incurred in manufacturing the battery monitoring apparatus.